PROJECT SUMMARY Radium-223 is the only FDA-approved alpha-emitting radionuclide. It is employed for the treatment of bone metastases in patients with castration-resistant prostate cancer. It is administered in the chemical form of the simple radium dichloride salt, which enables the unchelated calcium-mimetic radium(II) ion to localize to rapidly dividing bone metastases. Despite the significant therapeutic potential of this radionuclide, its current FDA-approved formulation cannot be applied for the treatment of soft-tissue metastases. The goal of this proposal is to expand the therapeutic potential of this nuclide to soft-tissue metastases by developing bifunctional chelating agents that can stably retain and deliver radium-223 in vivo. In Aim 1, we will synthesize novel diaza- 18-crown-6 macrocycles and calix[4]arene-based ligands as potential candidates for radium chelation. Our motivation for exploring these classes of ligands is dictated by literature precedence, which shows that these ligands are uniquely selective for large ions like radium(II). In Aim 2, we will collaborate with Prof. Babich at Weill Cornell Medicine to evaluate the radiochemistry of our best ligand candidates with radium-223. The radiolabeling kinetics and the stabilities of the resulting complexes will be measured. An ideal candidate for radium-223 chelation will rapidly and quantitatively form a complex with the radium(II) ion that is stable in serum over a period of at least 10 days. Based on these data, we will pursue the synthesis of bifunctional chelating agents in Aim 3. Using our most effective radium-223 chelating agent, we will synthetically install an amine-reactive isothiocyanate functional group onto the ligand to allow for its conjugation to soft-tissue tumor-targeting antibodies. This ligand-antibody conjugate will be radiolabeled with radium-223, and its in vivo biodistribution will be evaluated in tumor-bearing mice to test the long-term stability of our radiometal construct. Collectively, the successful execution of these three aims will lead to the discovery of new, bifunctional chelating agents for radium-223, which will expand the therapeutic use of this radionuclide to patients with soft-tissue metastases. Thus, this technology has the potential to significantly prolong and improve the lives of cancer patients by rendering access to the highly promising radium-223 radionuclide.